This application claims priority from Korean Patent Application No. 2003-92508, filed on Dec. 17, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical mode converter using omnidirectional reflectors, and more particularly, to an optical mode converter designed to minimize coupling loss through mode conversion between either an optic fiber or a laser diode and a waveguide through which light is transmitted.
2. Description of the Related Art
In recent years, there has been rapid growth in high-speed Internet access services offered via a communication network such as Asynchronous Digital Subscriber Line (ADSL) or Very-High-Data-Rate DSL (VDSL) by connecting between an Internet Service Provider (ISP)'s sending terminal and a subscriber's terminal via an optic fiber.
An optical transmitter-receiver (hereinafter transceiver) is essentially required for implementing an optical network. The optical transceiver converts a digitized electric signal to an optical signal within an arbitrary wavelength band for transmission through an optic fiber, and vice versa.
For the optical transceiver, it is very important to reduce coupling loss between an optic fiber that transmits external transmit/receive signals and an optical waveguide through which signals are transmitted. The waveguide is composed of a central core and a cladding layer surrounding the core. The core has a refractive index higher than that of the cladding layer. The total reflection due to the refractive index difference within the waveguide causes light to propagate through the core.
In this case, for a beam to propagate through the waveguide or optic fiber, the beam must meet certain propagation conditions. The beam is called a mode, and a mode size (beam diameter) is inversely proportional to a refractive index difference between the core and the cladding layer. A single mode optic fiber with low signal loss is commonly used for signal transmission. The single mode optic fiber typically has a large mode size of about 10 μm due to a small refractive index difference.
Since a mode size decreases as a refractive index difference between a core and a cladding layer increases, a waveguide can have a core with a small cross-section and realize a small bending radius due to low bending loss, thereby reducing the overall size and cost. For example, when the core is made of silicon (Si) with a refractive index of about 3.5 and the cladding layer is made of silicon dioxide (SiO2) with a refractive index of about 1.5, the waveguide has a refractive index difference greater than 130% and can realize a core size of sub-micrometers.
In general, a mode converter uses a tapered waveguide with a progressively increasing or decreasing width or thickness. However, for a high index contrast waveguide designed for a low price, ultraslim optical device, mode conversion cannot be achieved based on typical tapering since it has a very small mode size.
U.S. Patent Publication No. 2002/0118916 A1 has proposed a mode-size converter between a low index difference optic fiber and a high index contrast waveguide. The proposed mode converter includes a tapered low index difference waveguide with a width that gradually decreases for coupling with an optic fiber and a tapered high index contrast waveguide with a width that gradually increases in opposite direction in order to provide low loss coupling with the low index difference waveguide. The two waveguides are configured to overlap, so that a tapered region of the high index contrast waveguide is embedded within a tapered region of the low index difference waveguide.
The optical mode converter configured so that the two tapered regions overlap each other is not only difficult to actually realize due to an extremely complicated manufacturing process, but also has not sufficiently high optical coupling efficiency (75% coupling efficiency at wavelength of 1.55 μm)